This invention is concerned with improving the performance and reliability of shear-type electromagnetic locks.
Magnetic door latches have been employed in a variety of installations virtually since the discovery of magnetism. One such latch is disclosed in U.S. Pat. No. 2,673,755 granted Mar. 30, 1954 to H. L. Asp for xe2x80x9cMagnetic Door Catchxe2x80x9d. In that latch an armature hingedly carried on the door is attracted to and held by a permanent magnet mounted on the top wall of a cabinet near the door.
When more secure locking arrangements are required it is customary to employ an electromagnet which is energized to close the lock and de-energized to open the lock.
UK patent application GB2123472A published Feb. 1, 1984 discloses an electromagnetic door lock employing a relatively small and weak electromagnet. Movement of the door away from the magnet causes a hook to be swung into a rod to mechanically link the door to the magnet housing. The rather frail mechanism would appear not to be suitable for a secure application.
One type of electromagnetic lock which is fairly simple in construction and can be installed in a variety of ways is the shear-type electromagnetic lock. U.S. Pat. No. 4,826,223 granted May 2, 1989 to A. V. Geringer et al. for xe2x80x9cElectromagnetic Door Lock Devicexe2x80x9d discloses such a lock. In this device an electromagnet mounted in a door frame attracts, when energized, an armature loosely or flexibly mounted on the door and positioned to slide in alignment beneath the electromagnet when the door is closed.
In the shear-type lock the magnetic forces attracting the armature to the magnet are not sufficient to resist strong shear forces across the face of the magnet as the door is forced open. So these shear forces are opposed by some form of mechanical engagement between the armature and the magnet. In the locks disclosed in the Geringer et al. patent the mechanical engagement takes place when a member or members projecting from edges of either the magnet or the armature engage the other component when the armature moves into engagement with the magnet. This lock malfunctions, however, if the magnet is energized before the door is fully closed as the projection engages the other component before the magnet and armature are aligned and the door cannot be fully closed.
U.S. Pat. No. 6,007,119 granted Dec. 28, 1999 to T. E. Roth et al. for xe2x80x9cMulti-Directional Self-Aligning Shear Type Electromagnetic Lockxe2x80x9d seeks to solve the early energizing problem of Geringer by mounting the armature in a highly flexible, resilient mount and the projections on the armature are conical in configuration. The arrangement permits the armature to tilt and float over the face of the magnet until the projections become aligned with openings in the magnet.
A deficiency of the Roth et al. locks, which is shared incidently with the Geringer et al. locks, is that the flexible mounts for the armatures are weak and easily damaged. Such locks would not be suitable for applications where a great deal of security is involved.
Installations for storing classified military information require the highest degree of security and resistance to efforts to break into the installation.
This invention seeks to provide a shear-type electromagnetic lock which will function reliably under ordinary operating conditions, but securely resist being broken open by intruders.
In accordance with this invention there are two principal components as in other shear-type locksxe2x80x94an electromagnet assembly and an armature assembly. The magnet assembly comprises an electromagnet and an electromagnet holder including a component of a secondary locking system. The armature assembly comprises the armature, an armature holder including a second component of the secondary locking system, a pivotal mounting for the armature holder for movement from a first position away from the magnet to a second position near the magnet and means on the armature holder engagable with the magnet holder for moving the armature holder to its second position whereby the armature contacts the electromagnet when the armature and electromagnet are in alignment and the first and second components of the secondary locking system are aligned for possible engagement.
The components of the secondary locking system are designed to become operatively engaged when sufficient shearing force is applied to cause the armature to slide across the face of the electromagnet.
The lock further includes means on one of said assemblies for preventing engagement of said secondary locking components when the shear forces acting across the face of the electromagnet to defeat the lock do not exceed a predetermined amount and which permits engagement of the secondary locking components when the shear forces exceed the predetermined amount.